The prior art is illustrated in International patent application WO-A-90 12851 and European patent application EP-A-0 036 151.
Steam cracking is a basic process in the petrochemicals industry and consists of high temperature cracking then rapidly cooling a feed of hydrocarbons and steam. The principal operating problem arises from the deposition of carbon-containing substances on the internal walls of the facility. These deposits, constituted by coke or condensed, heavy pyrolysis tar, which is coagulated to a greater or lesser extent, limits heat transfer in the cracking zone (coils of pyrolysis tubes) and the indirect cooling zone (effluent transfer line exchanger), requiring frequent stoppages in order to decoke the facility.
Conventional cycle times (between two complete chemical decoking steps in the cracking zone, in air and/or steam) are either fixed (controlled stoppages) or variable depending on the coking in the facility, and are generally between 3 weeks and 12 weeks for feeds such as naphtha and liquefied petroleum gas.
The skilled person is aware that coking problems encountered when cracking heavy feeds (atmospheric gas oils, heavy gas oils, vacuum distillates) are far more severe than those encountered with conventional feeds such as naphtha.
As a consequence, these feeds cannot be cracked in conventional steam crackers designed for cracking naphtha, and they can only be cracked in existing processes in special furnaces typically comprising direct cooling (with pyrolysis oil) of the steam cracking effluents, which is considerably deleterious to the energy balance in the facility (no production of high pressure steam).
Known processes which are flexible as regards heavy feeds are thus incompatible with existing steam cracking facilities for conventional feeds and have a much worse energy balance.
We have proposed (EP-A-0 419 643, EP-A-0 425 633 and EP-A-0 447 527) a decoking process for use during the operation of steam cracking facilities by injection of solid erosive particles, to overcome coking problems and obtain continuous or substantially continuous steam cracking (for example with cycle periods of the order of one year).
For a particular feed, this process consists in allowing a layer of coke to form and age on the internal walls of the cracking coil, then injecting erosive particles (for example hard mineral particles with a diameter of less than 150 micrometers, which may be spherical or angular) in a sufficient quantity to substantially stabilise coking of the tubes without totally eliminating the precoat of coke which protects the tubes.
This process requires a good knowledge of the coking rates in the feed under consideration and a coil design which provides a certain amount of correspondence between the local coking rates connected to the progress of cracking along the coil and the erosion intensity connected to the rate profile along the coil and to the nature of the erosive particles. By means of simulations of coking rates and the circulation rate profile in the coil, and by means of pilot experiments, it is possible to produce substantially continuous steam cracking conditions in the feed under consideration.
Tube erosion can be maintained at a very low or zero level, and controlled by analysis of the trace metals (iron, chromium, nickel) in the recovered powder.
We have thus sought to perfect this process, which can be applied to cracking a particular feed, to a flexible furnace for the successive treatment of a large number of different feeds, under differing operating conditions (flow rate, dilution, cracking severity).
Pilot tests have been carried out and have produced several unexpected results:
Initial coking in the coil (at the beginning of the cycle) can vary very widely depending on the feed, even for feeds which only have slightly different compositions but are from different sources.
This cannot be fully explained, but may result from the presence of impurities in the feed.
Decoking efficiency has been shown to depend mainly on the feeds and operating conditions (different nature of coke). In particular, it has been found that light feeds: C.sub.3, C.sub.4, light naphtha, produce a catalytic coke at the beginning of the reaction zone which is much more fragile (5 to 10 times) than the asymptotic coke which predominates in the middle and at the end of the reaction zone. For these feeds, then it is desirable to limit the circulation rate in this zone to maintain the protective coke layer and/or avoid the risk of erosion of the cracking tubes.
Thus it has not been possible to predetermine the quantities of particles suitable for each feed and each operating condition without preliminary tests, which are impossible to carry out in the case of a flexible industrial furnace. Further, the geometry of the cracking reactor adapted for a given feed, as regards preventing erosion risks, is not the same as that adapted for another feed with a different dilution and different nature of coke (in which the circulation rate profile will be different).
Finally, because of the difficulties in obtaining reliable and precise measurements of the skin temperatures in the tubes by optical pyrometry, and the fluctuations in these temperatures and the pressure drop under varying operating conditions, it is very difficult to efficiently monitor coking in the tube without frequently reference to a constant reference state; this is not possible for a flexible industrial furnace and thus the coking state in a pyrolysis coil cannot be controlled in real time.
The process is thus difficult to carry out industrially under variable operating conditions, and it has not been possible to prevent all traces of erosion in the cracking tubes in the pilot tests.
It has thus become clear that the continuous steam cracking process cannot be adapted for a flexible furnace and must be reserved for cracking identical or near-identical feeds which are cracked under relatively stable conditions.
We have also shown that deposits in the transfer line exchanger can be eliminated far more easily than from the pyrolysis tubes and that, even in the case where an excess of particles is injected, no erosion occurs.
It has surprisingly become clear that carbon-containing deposits in the transfer line exchanger, in particular in the case of heavy feeds, are far more fragile than the coke in the cracking tubes. We have found that the fragility as regards erosion by the solid particles tested is at least 25 times greater for the coke in the transfer line exchanger than for the asymptotic coke in the pyrolysis tubes. The absence of erosion observed in the exchanger tubes themselves is explained by the fact that the circulation rate of the particles is much lower in the transfer line exchanger than in the pyrolysis tubes, and their temperature is very low (330.degree. C. as opposed to a typical value of 1000.degree. C. to 1100.degree. C. for the pyrolysis coil). Further, the transfer line exchanger tubes are straight, with no turns, which overcomes the risk of point erosion.
It has also become clear that over long periods, the coking rates in the cracking tubes remain of the same order of magnitude for heavy feeds (gas oil, for example) as for those of light feeds, and that the real obstacle to flexibility as regards heavy feeds resides in excessive fouling of the transfer line exchanger. Thus we have found, in a non obvious fashion, that existing naphtha steam cracking facilities can also be used to crack heavy feeds such as gas oils and vacuum distillates of the right quality, if rapid fouling of the transfer line exchangers can be prevented.
We are thus proposing a novel process for flexible steam cracking which is compatible with existing steam cracking facilities and which can treat a variety of feeds under various operating conditions, and without deteriorating the heat balance of the facilities, without notable risks of erosion and with moderate investment costs.